To collect biological or bioelectric signals (e.g., electrocardiographic (ECG), electromyographic (EMG), electroencephalographic (EEG), etc.), it is often necessary to attach electrodes either on the surface of a subject's skin or implanted under the subject's skin. FIG. 1 illustrates an example block diagram of a conventional system that may be used for recording and analyzing EEG signals for the primary purpose of medical diagnostics. Power supplies, patient isolation, and many other elements of a complete bioelectric signal-monitoring system are not depicted in this example system since such features are well known to those versed in the art.
As shown in FIG. 1, the system 100 employs two signal electrodes 110, 120 and one common electrode 130 all of which are attached to a subject from whom it is desired to receive EEG signals. The electrodes 110, 120, 130 are placed at particular locations depending on the intended application. It is common in clinical and research applications to place several signal electrodes at various locations on the subject's head to obtain EEG information associated with different regions of the brain in either a differential arrangement, which is shown, or in a referential arrangement. Electrodes are typically secured to the subject's scalp to provide a stable mechanical connection therewith and, often, conductive gel or paste is used to establish a good electrical connection with relatively low impedance.
The body generates low voltage EEG electrical signals (in the microvolt range) with relatively high output impedance. To obtain sufficient amplitude for recording or processing, these EEG signals must be amplified. Accordingly, the electrodes 110, 120 are connected to an EEG amplifier 140. The common electrode 130 is used to ensure that the amplifier 140 and subject are at the same electrical potential. This can significantly reduce common mode interference and stabilize the amplification process. The electrodes 110, 120 are connected to the EEG amplifier 140 using suitable leads, which are selected to minimize noise contamination. Leads may be provided with a shield as an additional noise reduction step, but this can increase capacitance and the loading effects must be carefully considered. The EEG amplifier 140 is typically a high-input-impedance linear amplifier having several user-selectable gain settings in the range of 103-105. A variety of EEG amplifiers are commercially available in various configurations. For example, Grass Technologies of West Warwick, R.I. sells a variety of clinical and research EEG amplifiers and systems.
The EEG amplifier 140 amplifies the differential signal between electrodes 110, 120 and produces an output voltage, Vamp, which is then filtered by, for example, a low pass filter 150 as shown. Then, the filtered output voltage is digitized by, for example, an A/D converter 160. The filtered and digitized signal is then typically fed to a digital signal processor (DSP) 170, processor, computer, etc. for numerical computations, storage, display, etc.
As shown in FIG. 1, the impedances at the junction between the subject's skin and the contact surface of the electrodes (i.e., skin-to-electrode impedances that are referred to hereinafter as “electrode impedances”) are indicated as Z1, Z2 and Zc for the two signal electrodes 110, 120 and the common electrode 130 respectively. The quality of the electrical connection between the body and the electrodes can have a significant impact on signal quality. That is, high electrode impedances and/or disparate electrode impedances can lead to poor signal quality. For good signal quality, the electrode impedances Z1, Z2 and Zc should be measured to ensure that they are balanced and within limits that are appropriate for the intended application.
Although various apparatuses, systems and methods exist for measuring electrode impedances, use of such conventional apparatuses, systems and methods is somewhat disadvantageous because they generally require manual intervention where the electrode leads are disconnected from the amplifier thereby interfering with the signal-monitoring/recording process. For example, the EZM® Electrode Impedance Meters from Grass Technologies of West Warwick, R.I. provide the ability to manually test applied electrodes at the subject site before connection to the recording instrumentation. However, if one or more electrodes were to experience a fault condition (e.g., by becoming partially or entirely disconnected from the subject) that goes unnoticed during monitoring/recording of bioelectric signals, the conventional apparatuses, systems and methods would not be able to detect and indicate the fault condition.
Furthermore, use of the conventional apparatuses, systems and methods is disadvantageous because they are unable to accurately measure impedances of a small number of electrodes individually. As known in the art, these conventional apparatuses, systems and methods either measure electrode impedances in a pair-wise manner, or measure a single electrode in series with the parallel combination of the remaining electrodes. With regard to conventional apparatuses, systems and methods using pair-wise measurements, these conventional apparatuses, systems and methods will just provide the impedance sum of each pair, not the individual impedance value of each of the electrodes. With regard to conventional apparatuses, systems and methods that measure a single electrode in series with the parallel combination of the remaining electrodes, it can be appreciated that these apparatuses, systems and methods assume a large array of electrodes which results in the parallel combination of electrode impedances of the remaining electrodes being very small in comparison to the electrode being measured so that the sum is dominated by the measurement electrode. With only a few (e.g., 2 or 3) electrodes in parallel, one cannot assume that these parallel electrodes have a very small impedance in comparison to the electrode impedance of the electrode being measured.
In view of the foregoing, apparatuses, systems and methods for determining electrode impedances, particularly at high speed and without having to disconnect the electrodes from an amplifier, would be an important improvement in the art. Additionally, apparatuses, systems and methods for determining electrode impedances of a small number of electrodes individually would be an important improvement in the art.